Fabrication and characterization of compound semiconductor sensors for pressure, gas, chemical, and biomaterial sensing

GaN-based diodes and high electron mobility transistors (HEMTs) for pressure, gas, liquid and biological sensing were fabricated and characterized. A novel metal oxide, ZnO was also evaluated as a potential electronic device for chemical sensing applications. The devices presented herein showed high sensitivity and capability of operation in harsh environmental conditions such as high temperature and pressure.; The AlGaN/GaN HEMTs grown on (0001) sapphire substrates show polarization-induced two dimensional electron gas (2DEG) at the AlGaN/GaN hetero-interface. Linear dependence of the 2DEG channel conductance on external strain was observed using a cantilever beam in a bending configuration. To overcome the rigidity of sapphire substrates in applying external stress, a micro pressure sensor using a 150 mum diameter thin flexible AlGaN/GaN circular membrane with an interdigitated-forger device on a (111) Si substrate was demonstrated. The measured pressure sensitivity was 7.1 x 10-2 mS/bar, which was two orders of magnitude larger than that of a cantilever beam pressure sensor.; Pt gated AlGaN/GaN HEMT-based metal-oxide semiconductor (MOS) diodes and field effect transistors (FETs) were demonstrated for detecting hydrocarbon gases, followed by a comparison between MOS and W/Pt Schottky-based GaN diodes for hydrogen sensing. Changes in current, when sensors exposed to hydrogen on the Pt-gated AlGaN/GaN HEMT were approximately an order of magnitude larger than that of Pt/GaN Schottky diodes and 5 times larger than Sc 2O3/AlGaN/GaN MOS diodes.; For the liquid sensors, gateless AlGaN/GaN HEMMs showed large changes in source-drain current on exposing the gate region to polar liquids and block copolymers. The polar nature of these chemicals leads to a change of surface charge in the gate region on the HEMT, producing a change in surface potential at the semiconductor/liquid interface. For biomaterials detection, the gate region was chemically modified with aminopropyl silane. As streptavidin was introduced to the biotin-functionalized gate region, the drain-source current showed a clear decrease of 4 muA, which shows interaction between antibody and antigen.; A Schottky diode was fabricated on a ZnO thin film and showed higher sensitivity to hydrogen (5 ppm). A single ZnO nanorod FET-based sensor was also demonstrated. Conductivity of the single nanorod sensor decreased linearly when the pH value of the solution varied from 2 to 12. The measured sensitivity was 8.5 nS/pH in the dark and 20 nS/pH under UV (365 nm) illumination, showing tremendous potential for sensing applications.